This invention relates generally to series compensated electric power lines, and more specifically concerns a system for preventing zone 1 protection for the line from operating for faults outside of the protected zone.
It is well known to use compensation in the form of series connected capacitors in a power line in order to transmit more power along the line. The amount of capacitance will determine the percentage of compensation for the otherwise inductive power line. Fifty percent (50%) compensation is a common example, which results in the power line looking 50% shorter electrically. The individual capacitors are lumped together in one location on the line. Because the capacitance reactance is lumped, there is a large discontinuity in the line impedance, i.e. the ohmic value per mile changes significantly for faults on either side of the capacitance.
Series compensation by adding capacitance in effect changes the appearance of the location of faults for conventional protective elements such as distance elements, which are typically used to determine the distance from a protective device such as a relay to the fault. Typically, a distance relay measures the impedance to the fault by using a phase angle comparator, measuring the angle between a polarizing or reference voltage V and a calculated voltage value [rxc2x7Zxc2x7I]xe2x88x92V, where r is a scalar value, and Z=R+jXLxe2x88x92jXC. I is the measured loop current and V is the measured loop voltage. If the angle between the two voltages is less than 90xc2x0, the fault is internal, while if the angle is greater than 90xc2x0, the fault is external. An angle of 90xc2x0 indicates the fault is on the boundary.
However, the presence of the series capacitance may cause the zone 1 protection of the relay to overreach, resulting in a potential misoperation. Accordingly, it is desirable to be able to block the trip operations of zone 1 protective elements until it is accurately determined that the fault is in fact between the relay and the series capacitor, i.e. the fault is internal to the line portion protected by the relay.
Accordingly, the present invention is a system for preventing overreaching of zone 1 protection for a series compensated power line, comprising: a comparator for comparing a ratio value of preselected power system voltage quantities with a preselected threshold ratio value; a timing device responsive to a comparator signal to produce a timer output signal for a limited time, when the ratio value is less than the threshold value; and an output element, when enabled, responsive to the timer output signal to produce a blocking signal for the protective elements for the power line.